1. Field of the Invention
This invention relates to a new process for the production of organic polyisocyanates containing at least partly blocked isocyanate groups using cyclic dicarbonyl compounds as blocking agents for isocyanate groups, to the polyisocyanates containing at least partly blocked isocyanate groups obtainable by this process and to their use as reactants for organic polyhydroxyl compounds in the production of polyurethane plastics, aqueous stoving lacquers or aqueous polyisocyanate solutions or dispersions.
2. Description of the Prior Art
Blocked polyisocyanates which react at elevated temperature with compounds containing Zerewitinoff active hydrogen atoms are known and are described, for example in Houben-Weyl, "Methoden der Organischen Chemie", Vol. XIV/2, pages 61-70, or in Z. W. Wicks, Jr. Progress in Organic Coatings 9, 3-28 (1981). Suitable blocking agents for isocyanates are, for example, phenols, caprolactam, oximes and CH-acid compounds such as acetoacetic acid esters and malonic acid dialkyl esters.
Conventional blocked polyisocyanates may be used in combination with polyfunctional compounds containing Zerewitinoff-active hydrogen atoms for the production of heat-hardenable plastics, particularly stoving lacquers and coatings. In that case, they are used primarily in the form of solutions in organic solvents.
In the interests of pollution control and to save petroleum-based organic solvents, there is increasing interest in the possibility of using stoving resins of the type in question in the form of aqueous dispersions or solutions. Accordingly, there has been no shortage of attempts to produce water-soluble or water-dispersible blocked polyisocyanates. To this end, some of the isocyanate groups in the polyisocyanates are irreversibly reacted with compounds containing ionic, potentially ionic or non-ionic-hydrophilic groups. The remaining isocyanate groups are "masked" with one or more of the above-mentioned blocking agents which are readily split off again at elevated temperature. This procedure is described, for example, in DE-OS No. 2,456,469 and in EP OS No. 12,348. The hydrophilic or potentially hydrophilic synthesis components used therein include, for example, hydroxy carboxylic acids, tertiary amines containing hydroxyl groups, hydroxy polyethylene oxides, hydroxy sulfonic acids and polyaminosulfonates.
This procedure is attended by the disadvantage that a considerable proportion of the NCO-groups in the polyisocyanates used is always consumed for the reaction with the hydrophilic synthesis components mentioned. In the blocked polyisocyanates, this proportion of the functionality is then no longer available for a thermal crosslinking reaction with reactants containing Zerewitinoff-active hydrogen atoms at the stoving temperatures normally applied. Accordingly, the hydrophilic, blocked polyisocyanates described have technical disadvantages by comparison with the corresponding blocked isocyanates which are applied from organic solution.
Now, the object of the present invention is to provide blocked polyisocyanates which may be applied not only from organic solution, but also from the aqueous phase, without any modifications which would lead to the technical disadvantages mentioned above being required for their production.
This object is achieved by the provisions of the process according to the invention which is described in more detail hereinafter and by the products obtained by that process which are also described in detail hereinafter.
The process according to the invention is based on the surprising observation that reaction products of certain, cyclic dicarbonyl compounds described in more detail in the following with organic polyisocyanates represents blocked polyisocyanates which react with compounds containing Zerewitinoff-active hydrogen atoms at relatively low stoving temperatures and, in addition, are soluble or dispersible in water in the presence of weak organic bases, such as for example tertiary amines. It was also surprising to find that the cyclic dicarbonyl compounds may be added not only onto aromatic isocyanates, but also onto aliphatic isocyanates. This is of particular technical importance to the use of the adducts in the field of lacquers and coatings on account of the fastness of light of aliphatic isocyanates.
Hitherto, only adducts of cyclic dicarbonyl compounds of the type essential to the invention with aromatic monoisocyanates have been known from the literature (U. Herzog, H. Reinshagen, Eur. J. Med. Chem. 10, 323 (1975)). According to these authors, it was not possible to produce corresponding adducts with aliphatic isocyanates. The object of the cited works was the production of pharmaceuticals and insecticides (see also U.S. Pat. Nos. 4,073,932 and 4,123,551). Adducts of polyfunctional isocyanates and the cyclic dicarbonyl compounds mentioned have never been described before. Likewise, nothing is known of the reactivity of adducts such as these to compounds containing Zerewitinoff-active hydrogen atoms or of attempts to use such adducts for applications in the plastics field.